The battery in a motor vehicle, which serves to supply the electrical systems and particularly also to supply the starter, is charged with the aid of a three-phase generator which is driven by the engine and whose output voltage is regulated in a suitable way. Since the battery must supply a multiplicity of consumers, it is possible under unfavorable operating conditions, for example given low temperatures and unfavorable driving conditions, for the battery to become discharged to such an extent that a reliable power supply is no longer guaranteed. In this case, starting the engine, in particular, can become problematical, since the starter requires considerable electrical energy.
It is therefore known to determine the energy content of the battery which is still available. A plurality of different methods have been carried out for this purpose; one of these methods consists in using measurement technology to determine the electrochemical battery state variables of acid density and the open-circuit voltage derived therefrom. However, this method is unreliable, since it would be necessary to take diffusion equalization times and temperature variations into account for a battery in operation.
A further method, which can be used, however, only in the steady state, consists in determining the internal resistance of the battery, which can serve as a measure of the charge state. The internal resistance can be determined in this case by observing the terminal voltage of the battery while a load pulse is applied to it.
A further possibility consists in determining the charge state of a battery, for example the starter battery of a motor vehicle, by continuously measuring the battery current and determining therefore the charge drawn from the battery by integrating over time. This mode of procedure is described in DE-OS 35 20 985. In accordance with the method described therein, the battery voltage is additionally measured after switching off the vehicle engine and a fictional battery capacity is computed from the reduction with respect to the voltage with a fully charged battery and the drawn charge calculated for the de-energized state. The charged state is then determined from this fictional battery capacity.
However, this method is still not yet reliable enough, since all battery systems have the unpleasant characteristic that the variation in capacity, and thus the residual capacity still available, referred to a defined final voltage, depends to a relatively large extent on a multiplicity of variables. Examples of these variables are the magnitude of the current drawn or supplied, the temperature, the age of the battery and the battery charge status, which has already been introduced.